Pomorzany Sewage Treatment Plant in Szczecin ensures the required parameters of treated sewage. However, due to higher efficiency of sewage treatment, more sludge is produced after the treatment process. In the examined sludge treatment plant, primary sludge is gravitationally thickened to the content of about 5% of dry matter, and the excessive is thickened in mechanical compactors up to 6% of dry matter. Settlements preliminary and excessive after compaction is discharged to the sludge tank where a pump is forced into two closed digesters. Each digester has the capacity of 5069 m³. At a temperature of about 37 °C a mesophilic digestion is performed. Biogas, that is produced in the chamber, is stored in two-coat tanks with the capacity of 1500 m³ each and after desulphurization with the biosulfex method (which results with obtaining elemental sulphur) it is used as fuel in cogeneration units. The aim of this study was to determine amount of energy given by sewage sludge in the form of heat during the process of methane digestion (primary and excessive). These amounts were determined on the basis of chemical energy balance of sewage carried into and out of Separate Sludge Digesters and produced biogas within 24h. The study determined that the percentage value of average chemical energy amount turned into heat and discharged with produced methane in relation to chemical energy of sewage carried into the first digester in Pomorzany Treatment Plant in Szczecin was in the range of 47.86 ± 9.73% for a confidence level of 0.95. On average 80.86 ± 33.65% was emitted with methane and 19.14 ± 33.65% of energy was changed into heat.